This invention relates to the formation of toner images on an image member. Although not limited thereto, the invention is particularly useful in a method and apparatus for forming two or more different color toner images on a single frame of an image member.
U.S. Pat. No. 4,546,060, Miskinis et al issued Oct. 8, 1985, discloses a method of developing electrostatic images using a developer including a "hard" magnetic carrier having a coercivity of at least 300 gauss when magnetically saturated and exhibiting an induced magnetic moment of at least 20 EMU/gm of carrier when in an applied field of 1000 gauss. A preferred embodiment of this carrier having a much higher coercivity in the neighborhood of 2000 gauss is commercially used to provide the highest quality electrostatic image development presently available. In this method, developer made up of such hard magnetic carrier particles and oppositely charged toner particles is moved at the speed and direction of the image by high s speed rotation of a magnetic core within a shell or sleeve on which the developer moves. Rapid pole transitions on the sleeve cause the high coercivity carrier to experience a torque. "Strings" or "chains" of the carrier rapidly flip on the sleeve to move the developer on the shell in a direction opposite to that of the rotating core. In contrast, a low coercivity, "soft" magnetic carrier will internally magnetically reorient in response to the pole transitions and not experience a torque adequate to cause the carrier to flip.
U.S. Pat. No. 5,001,028 to Mosehauer et al is representative of a number of references describing a process in which a photoconductive image member is uniformly charged and imagewise exposed to create an electrostatic image. Dry toner is applied to the electrostatic image to create a toner image. Usually in this process, discharged area development is used. Thus, the toner applied is of the same polarity as the electrostatic image. Deposits in the areas of lowest charge (the discharged areas) form a toner image having a density which is greatest in the portions of the image receiving the greatest exposure.
Without fixing the first toner image, the image member is usually uniformly charged, again with a charge of the same polarity as the original image and imagewise exposed to form a second electrostatic image generally in the portions of the image member not covered by the first toner image. The second electrostatic image is toned, again with a toner of the same polarity as the electrostatic image but of a color different from the first toner image, to create a second toner image. The process can be repeated with a third electrostatic image toned by a third color toner to create a three color image, etc. The two (or more) color images all have the same polarity and are easily transferred in a single step to a receiving sheet and fused, also in a single step.
Although the process is not necessarily limited to such applications, it is most commonly used to provide accent color prints or copies with laser or LED printhead electronic exposure. All commercial applications known to us use electronic exposure and discharged area development.
The process has a number of advantages in multiple color applications. It eliminates the troublesome, inaccurate and/or expensive steps used in registering images at a transfer station. If it uses separate exposure stations for each image, it can produce multiple color output at the same speed as single color output.
It is important that the second and subsequent toning steps not disturb the first toner image. Otherwise, toner from the first toner image gets mixed into the second development station ("scavenging") and toner from the second development station is deposited on the first toner image ("overtoning"). The relative seriousness of scavenging and overtoning is dependent upon the order of colors. In a system in which a lighter color is deposited first and a darker color later, overtoning is more serious than scavenging. However, in a system in which a darker color, for example, black, is deposited first and the lighter color is deposited second, scavenging of the dark color into the light color toning station is a much more serious problem. Note that overtoning often occurs as a result of scavenging with the second color replacing the scavenged first color toner.
Much of the art prior to Mosehauer recommends use of projection toning for the second and subsequent toning steps in order not to disturb the first image. Unfortunately, it is difficult to obtain reasonable density in high speed imaging with projection toning. The Mosehauer patent suggests that excellent results are obtained using the Miskinis method to develop the second and subsequent images. In its preferred embodiment, the nap of the brush is actually brought into contact with the image. This provides extremely high density images at high process speed. It provides far less scavenging than other high density, high speed systems because of an inherent softness in this type of brush. That is, it does not physically or mechanically rub off the first toner image. However, with this system some scavenging does occur. It remains much preferable to prior projection toning approaches in applications which require high density and high process speed.
Japanese Kokai 56-144452, published Nov. 10, 1981, shows a number of projection toning systems for toning electrostatic images in the presence of unfixed first toner images including rotating core magnetic brushes with two component developer.
U.S. Pat. No. 4,629,669 (Shoji et al) is one of a number of references showing the use of an alternating current field in order to effect projection toning of a series of electrostatic images. Some of the examples in this reference suggest the use of two component developers with a rotating magnetic core inside a rotating sleeve. This reference suggests many solutions to scavenging, including 1) increasing the magnetic field with later images; 2) increasing the amount of developer exposed to the image with later images; 3) increasing the toner concentration with later images; 4) increasing the original charge on the photoconductor with later images; 5) increasing the charge on the toners in later images; and 6) varying the AC component of the bias by reducing a high harmonic wave component for later images. All examples employ toner particles 10 microns in size or greater, although the reference indicates smaller toners would give better resolution. In general, the AC component of the bias is varied between 800 Hertz and 3 kHz with less good results at 800 Hertz. In the moving core examples, the sleeve is moved at or faster than the image, for example, 120-300 mm/sec, while the core is oppositely rotated at from 450 to 750 rpm. Best results were achieved with the developer moving two to three times as fast as the image. The carrier is quite insulating, preferably at 10.sup.14 ohm-cm.
U.S. Pat. No. 4,797,334, Hiratsuka issued Jan. 10, 1989, shows a rotating core and sleeve system in which a single electrostatic image is toned across a gap using a fast sleeve speed and an AC bias that is preferably between 300 Hertz and 2 kHz. The carrier is, again, quite insulating, preferably 10.sup.14 ohm-cm. U.S. Pat. No. 4,657,374, Kuramoto et al, has a similar disclosure for toning a single image. The core is rotated at between 1000 and 2000 rpm with a rapidly rotating sleeve (100-150 rpm).
U.S. Pat. No. 4,803,518 to Haneda et al granted Feb. 7, 1989 also shows development of an electrostatic image in the presence of an unfixed dry first toner image using a rotating core magnetic brush and a two component developer. The carrier typically has a resistivity of 10.sup.14 ohm centimeters and is 30 microns in size. Reduction of color mixing is accomplished primarily by adjustment of AC frequency, voltage and the development gap. The reference suggests that less scavenging will result if the toner has a high charge-to-mass ratio. It increases the toner charge-to-mass ratio with the later images, while reducing the amplitude and increasing the frequency of the AC component of the bias. It prefers projection toning at all stations, but notes that the first station can use contact toning. The toner has an average particle size of 10 microns. See also U.S. Pat. Nos. 4,666,804, Haneda et al issued May 19, 1987; 4,677,929, Haneda et al issued Jul. 14, 1987; 4,822,711, Itaya et al issued Apr. 18, 1989; 4,599,285, Haneda et al issued Jul. 8, 1986.
Those of the above references that suggest the second toning step in a multicolor system be carried out by projection toning with a rotating core brush and an AC component to the bias, generally call for moving the sleeve as fast or faster than the image to increase the amount of developer passing it. They appear to use high charge-to-mass toners to reduce scavenging. They use very insulating carriers. None of these references suggest the use of a high coercivity carrier.
Despite the disclosures in the above applications, the tradeoff between high density development and scavenging still exists in the above prior art. That is, in systems in which the developer nap is separated from the image member in order to reduce scavenging, the density of development makes the system less acceptable at high speed.